With an electrophotographic device or electrostatic recording device, a toner image is formed by depositing a toner on a latent electrostatic image formed on a photoconductor, transferring the deposited toner to a transfer material, and fixing the transferred toner with heat onto the transfer material such as paper. Moreover, a full-color image formation is generally a reproduction of a color with toners of four colors, black, yellow, magenta, and cyan, and form a full-color image by developing each color, and heating a toner image formed by laminating a layer of each toner on a transfer material, to fix the toner image.
The toner used in such image formation is desired to have excellent low temperature fixing ability corresponding to the development of the electrophotographic technology. For giving a toner low temperature fixing ability, a method where a glass transition temperature of a binder resin used in a toner is simply reduced is used, but this adversely affect storage stability of the toner.
Accordingly, various attempts have been made to use a polyester resin, which has high compatibility and excellent low temperature fixing ability compared to a styrene-based resin generally used as a binder resin of a toner in the conventional art, in a toner.
For example, as proposed toners, there are a toner containing a linear polyester resin whose physical properties, such as a molecular weight is specified (see PTL1), and a toner containing a non-linear crosslinked polyester resin using rosins as an acid component (see PTL2).
The toner containing the polyester resin using the rosins as mentioned has excellent low temperature fixing ability, as well as excellent grindability, and therefore it has an advantage that the toner production thereof by a pulverization method can be improved.
Moreover, by using 1,2-propanediol, which is C3 branched chain alcohol, as an alcohol component of the polyester resin, the low temperature fixing ability of the resulting toner can be improved while maintaining offset resistance of the toner, compared to the case where C2 or lower alcohol is used. Compared to the case where a C4 or higher branched alcohol is used, use of such an alcohol is more effective in preventing the storage stability from being deteriorated along with the reduction in the glass transition temperature.
By using the aforementioned polyester resin as a binder resin for a toner, the resulting toner has effects that fixing can be performed at low temperature, and that storage stability thereof improves.
Recently, there have been trends to further increase processing speed of image forming apparatuses, and to saving more energy required for operating image forming apparatuses. Under such circumstances, the conventional techniques mentioned above are not sufficient. Specifically, with the conventional techniques, it is extremely difficult to maintain sufficient fixation strength, i.e., image maintainability, because of reduced fixing duration during a fixing process, and lowered heating temperature by a fixing unit.
To solve these problems, there has been proposed a technique where low temperature fixing ability of a toner is improved by introducing as a fixing auxiliary substance, a plasticizer compatible with a resin during heating (see PTL3).
This proposed technique achieves both heat resistance storage stability and low temperature fixing ability of a toner by designing the toner to have the fixing auxiliary substance as a crystalline domain.
However, this technique has a problem that the resulting toner does not have sufficient image maintainability.
Moreover, there has been disclosed a toner which achieves both heat resistance storage stability and low temperature fixing ability by introducing a crystalline polyester resin therein (see PTL4, and PTL5).
However, with the proposed technique, the glass transition temperature of the toner reduces after heating, because of the introduced crystalline polyester resin, which adversely affects image maintainability of the toner.
Accordingly, there is currently a demand for a toner having excellent low temperature fixing ability, heat resistance storage stability, and image maintainability, as well as a developer, image forming apparatus and image forming method all of which use such a toner.